Generally, austenitic stainless steel products such as screws have been widely employed because of their superiority in not only corrosion resistance, but also toughness, workability, heat resistance and non-magnetic property and the like. However, although austenitic stainless steel products have excellent anti-corrosion property, as mentioned above, they do not have quenching hardenability so that they are not suitable for such usage as requires high surface hardness.
Among stainless steel materials, martensitic stainless steel containing chromium at 13 to 18% (by weight; the same applies hereinafter) have been also employed besides the above austenitic stainless steel. This martensitic stainless steel has quenching hardenability, however, it is inferior greatly in its corrosion resistance property to austenitic stainless steel. Therefore, this material cannot be applied to the such usage as requires corrosion resistance. On the other hand, from the viewpoint that the above austenitic stainless steel lacks surface hardness, hard chrome plating and the like has been applied in order to improve the deficiency. However, there is a problem practically in the above plating because the adhesive of plating coat is low thereto.
Recently, corrosion resistance of stainless steel has been focused on. It has been increasingly demanded to maintain this corrosion resistance and at the same time to improve its surface hardness. For this purpose, the inventors of the present invention have tried to apply nitriding treatment to austenitic stainless steel superior in corrosion resistance (18-8 stainless steel containing 18% chromium and 8% nickel has been widely employed) and to form a nitrided hard layer so as to improve its surface hardness.
As a method of nitriding treatment, a variety of methods such as salt bath nitriding treatment, ionitriding treatment, gas nitriding and the like are available. Nitriding temperature is usually set around 550.degree. to 570 .degree. C., and around 480.degree. at the lowest in these nitriding treatments. As results of nitriding screws made of austenitic stainless steel by such nitriding methods, in spite that the surface hardness has been improved, original property of corrosion resistance for stainless steel deteriorates so that a defect of easily rusting has been caused.